1. Field of the Invention
The present invention relates to an apparatus and a method for producing a catalyst, and a method for producing an unsaturated acid or an unsaturated nitrile.
2. Description of the Related Art
Previously, a method of subjecting propane or isobutane to a vapor-phase catalytic oxidation or vapor-phase catalytic ammoxidation reaction, so as to produce a corresponding unsaturated carboxylic acid or unsaturated nitrile, has attracted attention, and various types of oxide catalysts preferably used in such production have been proposed.
As a method for producing an oxide catalyst containing a Mo compound, a V compound and a Nb compound, there has been known a technique for obtaining an oxide catalyst by preparing an aqueous mixed solution, then spray-drying this mixed solution and then calcining it. In a mixed solution consisting of niobium and multiple types of other metal ions, individual metal ions have each different pH stable ranges. Thus, such a mixed solution has extremely low stability as a solution, and precipitation easily takes place. In particular, in a case in which an aqueous mixed solution contains Mo—V—Nb—(Te/Sb), such an aqueous mixed solution has a characteristic that it extremely easily becomes gelatinous under non-stirred conditions. Hence, there may be a risk that the composition of ingredients may become uneven in a local region in which stirring is insufficient, and that a decrease in catalytic performance may thereby occur. In addition, there may also be a case in which it becomes impossible to supply the prepared aqueous mixed solution to a spray dryer.
In order to prevent the aforementioned phenomenon, for example, Patent Document 1 describes a method for supplying an aqueous mixed solution to a pipe by controlling a retention time in a preparation step and/or a drying step. Moreover, Patent Document 2 discloses a method for controlling a non-stirring time in the preparation step. Furthermore, Patent Document 3 states that a stirring power is controlled within a constant range during the preparation step.